while-else-loop | 易学教程

问题

Of course this is an impossible statement in java (to-date), however ideally I would like to implement it as it is at the heart of many iterations. For example the first multiple times it is called I'm doing it 650,000+ times when it is creating the ArrayList. Unfortunately the reality is that my actual code does not have the set inside the else loop; thus it will pass over both the add and then the set commands and wasting time.

After that I have it also in another loop where it is only performing the set as the data is already created and this is multi-nested with in many others so it is a lengthy process.

ArrayList<Integer>  dataColLinker = new java.util.ArrayList<Integer>();
...
...
public void setLinkerAt( int value, int rowIndex) {
    ...
    while(rowIndex >= dataColLinker.size()) {
        dataColLinker.add(value);
    } else {
        dataColLinker.set(rowIndex, value);
    }

Any ideas or theories? I'm unsure about speeds in java when it comes to if statements and ArrayList commands and so on

回答1:

Am I missing something?

Doesn't this hypothetical code

while(rowIndex >= dataColLinker.size()) {
    dataColLinker.add(value);
} else {
    dataColLinker.set(rowIndex, value);
}

mean the same thing as this?

while(rowIndex >= dataColLinker.size()) {
    dataColLinker.add(value);
}
dataColLinker.set(rowIndex, value);

or this?

if (rowIndex >= dataColLinker.size()) {
    do {
        dataColLinker.add(value);
    } while(rowIndex >= dataColLinker.size());
} else {
    dataColLinker.set(rowIndex, value);
}

(The latter makes more sense ... I guess). Either way, it is obvious that you can rewrite the loop so that the "else test" is not repeated inside the loop ... as I have just done.

FWIW, this is most likely a case of premature optimization. That is, you are probably wasting your time optimizing code that doesn't need to be optimized:

For all you know, the JIT compiler's optimizer may have already moved the code around so that the "else" part is no longer in the loop.
Even if it hasn't, the chances are that the particular thing you are trying to optimize is not a significant bottleneck ... even if it might be executed 600,000 times.

My advice is to forget this problem for now. Get the program working. When it is working, decide if it runs fast enough. If it doesn't then profile it, and use the profiler output to decide where it is worth spending your time optimizing.

回答2:

I don't see why there is a encapsulation of a while...

Use

//Use the appropriate start and end...
for(int rowIndex = 0, e = 65536; i < e; ++i){        
    if(rowIndex >= dataColLinker.size()) {
         dataColLinker.add(value);
     } else {
        dataColLinker.set(rowIndex, value);
     }    
}

回答3:

Wrap the "set" statement to mean "set if not set" and put it naked above the while loop.

You are correct, the language does not provide what you're looking for in exactly that syntax, but that's because there are programming paradigms like the one I just suggested so you don't need the syntax you are proposing.

回答4:

boolean entered = false, last;
while (( entered |= last = ( condition ) )) {
        // Do while
} if ( !entered ) {
        // Else
}

You'r welcome.

回答5:

Java does not have this control structure.
It should be noted though, that other languages do.
Python for example, has the while-else construct.

In Java's case, you can mimic this behaviour as you have already shown:

if (rowIndex >= dataColLinker.size()) {
    do {
        dataColLinker.add(value);
    } while(rowIndex >= dataColLinker.size());
} else {
    dataColLinker.set(rowIndex, value);
}

回答6:

This while else statement should only execute the else code when the condition is false, this means it will always execute it. But, there is a catch, when you use the break keyword within the while loop, the else statement should not execute.

The code that satisfies does condition is only:

boolean entered = false;
while (condition) {
   entered = true; // Set it to true stright away
   // While loop code


   // If you want to break out of this loop
   if (condition) {
      entered = false;
      break;
   }
} if (!entered) {
   // else code
}

回答7:

Assuming you are coming from Python and accept this as the same thing:

def setLinkerAt(value, rowIndex):
    isEnough = lambda i: return i < dataColLinker.count()
    while (not isEnough(rowIndex)):
        dataColLinker.append(value)
    else:
        dataColLinker[rowIndex] = value

The most similar I could come up with was:

public void setLinkerAt( int value, int rowIndex) {
    isEnough = (i) -> { return i < dataColLine.size; }
    if(isEnough()){
        dataColLinker.set(rowIndex, value);
    }
    else while(!isEnough(rowInex)) {
        dataColLinker.add(value);
    }

Note the need for the logic, and the reverse logic. I'm not sure this is a great solution (duplication of the logic), but the braceless else is the closest syntax I could think of, while maintaining the same act of not executing the logic more than required.

来源：https://stackoverflow.com/questions/10791030/while-else-loop

标签

java

performance

arraylist

while-loop

theory